The prosthetic limb and foot devices which have been available have either severely limited the mobility of the user, or have sought to reconstruct the complex foot and ankle structure through the use of very complex prosthetic structures to simulate a human gait. Unfortunately, a significant need for prosthetic limbs has been created in underdeveloped countries by recent conflicts and the widespread use of landmines. In these places having the greatest need, the use of complex prosthetic devices is made impossible by the high cost. Only the simplest prosthetic designs are affordable. The lack of motion in the foot of these designs, such as the SACH (solid ankle cushion heel) foot or the Jaipur foot, cause restrictions to mobility, particularly over rough terrain. The cost of the SACH foot and the long fabrication time of the Jaipur foot have meant that these devices are unable to address the current need. In addition to high cost and poor mobility, prior prosthetic designs have been susceptible to damage by water, and to material fatigue over undesirably short periods.
A prosthetic foot which attempts to simulate a human gait is disclosed in U.S. Pat. No. 4,645,509 issued to Model and Instrument Development Corporation Feb. 24, 1987. This device includes a resilient cantilever spring bonded to a surrounding low density material. The cantilever spring is designed to provide energy return, particularly for active use. Impact at the heel is reduced due to the low density material designed to provide heel cushioning. Flexure of the cantilever spring during plantar flexion is limited by the rigidity of the spring material. The end of the cantilever element is upturned, corresponding to the ball of a foot, to reduce stress on the keel during toe off. This cantilever device does not provide a stable structure for toe off. The design necessarily causes strain between the high and low density materials, in addition to the complexity and cost of manufacture. A further difficulty of this design is that the stiffness appropriate for energy return during active use may be too resistant for normal walking.
U.S. Pat. No. 4,555,817 issued to Roderick W. McKendrick incorporates flexibility by interconnecting a plurality of elements with a cable. A compressible rubber block is positioned between substantially rigid elements in the heel region to add flexibility and to provide resilience to return the elements to their original positions. The tension of the cable can be adjusted to permit more or less flexibility for rotation, lateral, and medial movements and plantar and dorsiflexion. Numerous parts must be assembled adding significant expense, and the flexibility imparted by wood, leather and rubber parts are subject to wear and hardening. In addition, plantar flexion is created by the loose connection of the cable and relative sliding movement. The range of plantar flexion or dorsiflexion is no greater than permitted lateral movements and thus normal movement for walking is restricted.
A further example of a flexible prosthesis is disclosed in U.S. Pat. No. 5,219,365 issued to Sabolich Inc., Jun. 15, 1993. Flexibility is primarily created by a highly arched keel. Additional flexibility for plantar flexion and dorsiflexion is introduced by a horizontal slot in the ankle portion. The keel is under constant pressure putting significant stress on the prosthetic material. The heel and toe portions spread apart with the pressure of each step causing an unnatural motion in the foot, and significant strain to the cosmetic covering. The keel design does not ever provide a stiff platform from which to toe off. The horizontal slot for increasing plantar flexion and dorsiflexion mobility is positioned in the rear of the ankle portion. As a result, no stop limits the dorsiflexion range to an appropriate toe off position.
U.S. Pat. No. 4,792,340 issued to Ernest M. Burgess in 1988 discloses a prosthetic ankle having a precisely formed kerf as a flexure joint. The flexibility of the ankle joint is very limited, intending to be used with a prosthetic foot having enhanced flexure characteristics. The kerf includes a double convolute section the surfaces of which contact each other to limit movement. The kerf provides narrow controlled limits to plantar flexion, dorsiflexion, axial rotation, inversion and eversion. Once the limit is met, the ankle structure becomes rigid in each direction and provides no further cushioning. In addition, the kerf including double convolute curves is difficult to manufacture, and on its own cannot provide the flexibility needed for a normal gait, particularly over uneven ground. The costs of combining a complex foot design and a complex ankle design are out of reach of the majority of patients in need of a prosthetic limb.
It is an object of the invention to provide a simple and economical foot prosthesis which simulates human movement by providing a controllable range of plantar flexion, dorsiflexion, inversion and eversion. Flexibility in the foot provides significant advantages in mobility over a simple rolling motion. The prosthesis is advantageously light weight and resistant to wear.